The recent development of the advanced information society has raised the issue that weak electromagnetic radiation emitted from various types of communication equipment and electronic equipment causes malfunctions (electromagnetic interference (EMI)) in instruments and other components used in television sets and radios, communication equipment, medical equipment, ships, aircrafts, and so on, and there has been a need to establish international regulations on electromagnetic radiation emission. Accordingly, manufacturers of communication equipment of various types, personal computers, and the like generating noise that can cause EMI has been required to accurately measure the noise from their electronic components so as to take measures for EMI. That is, accurate measurement of very weak electromagnetic radiation from electronic equipment, etc. with a high performance instrument and measures to prevent emission of harmful electromagnetic radiation are being demanded. The problem encountered here is the environment in which electromagnetic radiation is measured. In order to accurately measure very weak electromagnetic radiation, a high performance anechoic chamber free from any extraneous disturbance such as noise is needed.
Electromagnetic radiation absorbers (hereinafter referred to as wave absorber(s) or simply absorber(s)) conventionally used in anechoic chambers are integrally molded structures having, as illustrated in FIG. 1, a plate-shaped part 1 and a polygonal pyramidal or wedge-shaped parts 2. These wave absorbers are dielectric loss structures made by expansion-molding an electromagnetic radiation absorbing material comprising a synthetic resin, such as polyurethane, polystyrene, or polypropylene, and an electrically conductive material, such as carbon. A hybrid wave absorber composed of a ferrite tile and the wave absorber of the dielectric loss type placed on the ferrite tile is known to exhibit good radiation absorption characteristics even with a reduced height of the dielectric loss structure because electromagnetic waves in a high frequency range of 100 MHz or higher are absorbed by the dielectric structure while electromagnetic waves in a low frequency range are absorbed by the ferrite tile, i.e., a magnetic absorber. With this hybrid wave absorber, there is provided an anechoic chamber of smaller size.
A number of proposals have been made with respect to polygonal pyramidal wave absorbers. For example, JP 2000-188513A discloses a wave absorber composed of a ferrite tile and an upper absorber having a wedge or pyramidal shape, the upper absorber being a molded part comprising a general purpose type resin having a relative permittivity of 4.9 or less at a frequency of 1 MHz or higher and ferrite powder dispersed in the resin. The wave absorber is described as being capable of absorbing higher frequency waves with the height of the upper absorber being equal to that of conventional wave absorbers. JP 2000-188513A also discloses a panel having four pyramidal parts the bottom of which are integrally connected to each other, the panel having a portion for screwing (e.g., a through-hole) at the center and on each of the four side edges thereof so that the absorber may be firmly fixed to construct an anechoic chamber with ease.
The structure of JP 2000-188513A shows excellent absorption characteristics for normal incidence of electromagnetic radiation. In practical electromagnetic radiation measurement, however, electromagnetic waves are incident on the ceiling and side walls of an anechoic chamber at a certain angle. For example, the angle of incidence is about 40° in a 10 m anechoic chamber. To meet the recent demand for a smaller-sized high-performance anechoic chamber, a wave absorber having absorption performance for oblique incidence as well as normal is required. The pyramidal shape of the absorber described in JP 2000-188513A has all the faces at the same angle of inclination, which is designed with weight put on normal incidence without due considerations given to oblique incidence characteristics.
In order to provide the structure of JP 2000-188513A with through-holes for screwing, adjacent pyramidal parts 2 must be arranged on the plate-shaped part 1 at a predetermined space, and a flat portion should be provided at the center of the panel as shown in FIG. 1. Such an arrangement of wave absorbers involves reduction of absorbing performance due to reflection on these flat portions, and the reduction of absorbing performance should be compensated for by, for example, increasing the height of the pyramidal parts 2.